What are photons? How can they affect reactions?

Has anyone seen photons? (I mean: Is there a sufficient proof of their existence?)

Is there a correlation between the amount of observers and the impact of the observation on the reaction that is being observed?

Please answer my questions in a simple way like you did in your first reply. Remember, I'm used to think that when you 'observe' anything, you can see only the light rays which have "bounced" back from it. I also don't accept the quantum theories, firstly because they don't seem to explain anything in the "real" world.

"Has anyone seen photons?"